Tissue cultures in the study group will be divided into 2 groups; The 1st group will be composed of 40 tissue culture plates and will be exposed to the total fluence dose 4.2 J/cm2 in a single session of 16 minutes duration with low level therapy machine adjusted at power of 40mw. The 2nd group will be composed of another 40 tissue culture plates, they will be exposed to the same fluence rate but in multiple sessions ''3 sessions'', which will be performed every other day, each session will last for approximately 5.34 minutes using low level laser of power of 40mw to reach the same fluence dose of 4.2 J/cm2 after finishing the 3rd session.